The present invention relates to an electromagnetic transponder, that is, a transceiver (generally mobile) likely to be interrogated contactless and wireless by a unit (generally fixed), called a read and/or write terminal. The present invention more specifically relates to transponders having no independent power supply. Such transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read/write terminal. The present invention applies to such transponders, be they read-only transponders, that is, transponders adapted to operating with a terminal which only reads the transponder data, or read/write transponders, which contain data that can be modified by the terminal.
Systems using electromagnetic transponders are based on the use of oscillating circuits, including a winding forming an antenna, on the transponder side and on the read/write terminal side. These circuits are intended to be coupled by a magnetic field when the transponder enters the field of the read/write unit.
FIG. 1 very schematically shows a conventional example of a data exchange system between a read/write unit 1 and a transponder 10 of the type to which the present invention applies.
Generally, terminal 1 is essentially an oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1, between an output terminal 2 of an amplifier or antenna coupler (not shown) and a reference terminal 3. The antenna coupler belongs to a circuit 4 for controlling the oscillating circuit and exploiting received data including, among others, a modulator-demodulator and a microprocessor for processing the control signals and the data. In the example shown in FIG. 1, node 5 of the connection of capacitor C1 to inductance L1 forms a terminal for sampling a data signal received from the demodulator. Circuit 4 of the terminal generally communicates with different input/output circuits (keyboard, screen, means of transmission to a provider, etc.) and/or processing circuits, not shown. The circuits of the read/write terminal draw the power required by their operation from a supply circuit (not shown) connected, for example, to the electric supply system.
A transponder 10, intended for cooperating with a terminal 1, essentially includes a parallel oscillating circuit formed of an inductance L2, in parallel with a capacitor C2 between two input terminals 11, 12 of a control and processing circuit 13. Terminals 11, 12 are in practice connected to the input of rectifying means (not shown), the outputs of which form D.C. supply terminals of the circuits internal to the transponder. Since transponder 10 draws its power from the field radiated by terminal 1, it is necessary to provide means 14 for limiting the input voltage of circuit 13 that would otherwise risk being damaged by too high voltages. Means 14 are symbolized in FIG. 1 by a zener diode forming means for clipping the voltage across capacitor C2. Means 14 are shown to be in parallel with capacitor C2. It should however be noted that any other equivalent means may be used and that the clipping means may be placed downstream of the rectifying means.
The oscillating circuit of terminal 1 is excited by a high-frequency signal (for example, 13.56 MHz) intended to be sensed by a transponder 10. When transponder 10 is in the magnetic field of terminal 1, a high-frequency voltage is generated across terminals 11, 12 of the transponder""s resonant circuit. This voltage, after being rectified, is intended to provide the supply voltage for the electronic circuits 13 of the transponder. These circuits generally essentially include a microprocessor, a memory, a demodulator of the signals possibly received from terminal 1, and a modulator for transmitting information to the terminal.
The oscillating circuits of the terminal and of the transponder are generally tuned on the frequency of a transmission carrier, that is, their resonance frequency is set to a frequency of, for example, 13.56 MHz. This tuning aims at maximizing the power diffusion to the transponder, generally, a card of credit card format or a tag of still smaller format, integrating the different transponder components.
The high-frequency remote supply carrier transmitted by the terminal is also used as a data transmission carrier. This carrier is generally modulated in amplitude by the terminal according to different coding techniques to transmit the data to the transponder.
FIG. 2 illustrates a conventional example of a data transmission from terminal 1 to a transponder 10. This drawing shows an example of a shape of the excitation signal of antenna L1 for a transmission of a code 0101. The modulation currently used is an amplitude modulation with a 106-kbit/s rate (1 bit is transmitted in approximately 9.4 xcexcs) much smaller than the frequency (for example, 13.56 MHz) of the carrier coming from the transmission oscillator (period of approximately 74 ns). The amplitude modulation is generally performed with a modulation rate (defined as being the difference of the peak amplitudes (a, b) between two states (1 and 0) divided by the sum of these amplitudes) much smaller than one due to the need for supply of transponder 10. In the example of FIG. 2, the 13.56-MHz carrier is modulated in amplitude, with a 106-kbit/s rate, with a modulation rate tm of, for example, 10%. It should be noted that, whatever the type of modulation used (for example, amplitude, phase, or frequency modulation) and whatever the type of data coding (NRZ, NRZI, BPSK, Manchester, ASK, etc.), the transmission is performed by jumps between two binary levels on the remote supply carrier.
A disadvantage of conventional transponders is that the use of means for clipping the voltage recovered across the oscillating circuit (L2, C2, FIG. 1) is incompatible with an amplitude shift keying that is not in all or nothing. Indeed, if the transponder is relatively close to the terminal, the voltage is likely to be clipped in such a way that the transponder demodulator is then incapable of making out a state 0 from a state 1 due to the modulation rate used. Further, this loss of information can occur without having a clipping level lower than the level of state 0 (b, FIG. 2). It is indeed sufficient for the level at state 1 to be clipped to have a risk of interpretation error by the transponder demodulator.
A known solution to solve this problem consists of limiting the transmission power of the terminal so that a transponder located very close to the terminal does not receive a voltage such that its clipping means are active. A disadvantage of such a solution however is that this then limits the range of the transponder system.
Further, the magnetic fields that the transponders are supposed to withstand are most often imposed by industry or government standards and the application of the standards now in force results in a magnetic field received by the transponder, when its clipping means operate, which is much smaller than the maximum magnetic field that the transponder must be able to withstand according to the standards.
The above problems are more critical still for low power consumption transponders that are tuned on the resonance frequency. Indeed, in such a case, the circuits internal to the transponder provided to have a low consumption are not able to withstand high voltages, so that the clipping means must be sized accordingly.
The embodiments of the present invention overcome the disadvantages of known electromagnetic transponders as concerns the unwanted effects of the clipping means.
The embodiments of the present invention more specifically aim at providing a novel electromagnetic transponder that can withstand high magnetic fields in the vicinity of a read/write terminal without adversely affecting the recovery of the data transmitted by this terminal. This requires no modification of the read/write terminals and that is thus compatible with existing read/write systems.
The embodiments of the present invention provide a solution that is compatible with the search for a minimum transponder power consumption.
To achieve the foregoing, the present invention provides an electromagnetic transponder of the type including a parallel oscillating circuit adapted to extracting a supply signal from a radiated field, at least one detuning element controllable from the oscillating circuit, and means for measuring a voltage depending on the voltage level recovered across the terminals of the oscillating circuit, and for activating the detuning element when this voltage exceeds at least a predetermined activation threshold.
According to an embodiment of the present invention, the detuning element is formed of a secondary capacitor adapted to being associated in parallel with a main capacitor of the oscillating circuit.
According to an embodiment of the present invention, the secondary capacitor is made controllable by being associated in series with a switch across the terminals of the parallel oscillating circuit.
According to an embodiment of the present invention, the transponder includes at least two individually controllable detuning elements, each detuning element being associated with a predetermined activation threshold dedicated thereto, for being activated when said voltage exceeds this threshold, so that the detuning of the oscillating circuit is performed in stages as the voltage level recovered across the terminals of the oscillating circuit increases.
According to an embodiment of the present invention, the deactivating of the detuning elements is performed simultaneously for all detuning elements, when said voltage falls back under a predetermined deactivation threshold.
According to an embodiment of the present invention, said measurement means are formed with a circuit including, in cascade, a number of measurement stages corresponding to the number of detuning elements plus one.
According to an embodiment of the present invention, all measurement stages have identical structures and receive, on a measurement input terminal, said voltage lowered by a predetermined amount different for each stage.
According to an embodiment of the present invention, said circuit provides control signals of activation and deactivation of the detuning elements via bistable circuits.
According to an embodiment of the present invention, the transponder includes means for clipping the voltage across the terminals of the oscillating circuit, the triggering threshold of which is greater than the activation threshold of the detuning element having the highest threshold.